An implant is understood in general to be any mechanical device (whether including moving parts or not) made of one or more materials which is intentionally introduced into the body and is covered either partially or completely by an epithelial surface. Implants can be subdivided into temporary and permanent implants with regard to the duration of use. Temporary implants remain in the body for a certain period of time. Permanent implants are intended to remain in the body permanently. Implants can also be differentiated according to prostheses and artificial organs. A prosthesis is a medical device which replaces extremities, organs or tissues of the body, whereas an artificial organ is understood to be a medical device that partially or completely replaces the function of an organ of the body. Implants such as orthopedic or osteosynthetic implants, cardiac pacemakers, defibrillators and vascular implants, for example, fall under the definitions given above.
An implant material is a nonviable material, which is used for an application in medicine and enters into interactions with biological systems. Biocompatibility is the basic prerequisite for use of a material as an implant material coming in contact with the biological environment when used as intended. Biocompatibility is understood to be the ability of a material to induce an appropriate tissue reaction in a specific application. This includes an adaptation of the chemical, physical, biological and morphological surface properties of an implant to the recipient tissue with the goal of a clinically desired interaction. The biocompatibility of the implant material also depends on the chronological course of the reaction of the biosystem in which it is implanted. Thus relatively short-term irritation and inflammation occur, possibly leading to tissue changes in the medium to long range, toxicity, allergies or even cancer by the results of inadequate biocompatibility.
The response of biological systems to foreign bodies depends on the properties of the material or component in a variety of ways. According to the response of the biosystem, the implant materials may be subdivided into bioactive, bioinert and degradable/resorbable materials.
For the purposes of the present invention, metallic implant materials consisting entirely or in part of magnesium or biocorrodible magnesium alloys are of interest, their application being in osteosynthesis, joint replacement, dental surgery and vascular surgery, for example.
One problem with the use of biocorrodible magnesium alloys is the rapid degradation of the material in a physiological environment. The principles of magnesium corrosion as well as some technical methods for improving corrosion properties (in the sense of strengthening the protection against corrosion) are known from the prior art. Such known methods, however, have unresolved problems and disadvantages associated with them.
Methods of creating a corrosion-preventing layer have not been developed for medical technical use of biocorrodible implants in a physiological environment.
Traditional technical fields of use of molded bodies of magnesium alloys outside of medical technology usually require extensive suppression of corrosive processes. Accordingly, the goal of most known technical methods is to completely eliminate corrosive processes. However, such methods may not be desirable for medical implant applications. Furthermore, for a medical technical use, toxicological aspects must also be taken into account. Furthermore, corrosive processes depend greatly on the medium in which they take place, and therefore findings obtained about corrosion prevention in the technical field under traditional ambient conditions should not be applicable to an unlimited extent to the processes in a physiological environment. Finally, with a variety of medical implants, the mechanisms underlying the corrosion could also deviate from conventional technical applications of the material. For example, stents, surgical material or clips undergo mechanical deformation during use, so the partial process of stress corrosion cracking should play a major role in the degradation of these molded bodies.
The basic body of some implants such as stents in particular is subject to plastic deformation of different intensities locally during use. Conventional methods for inhibiting corrosion, e.g., generation of a dense magnesium oxide cover layer, are not helpful here. The ceramic properties of such a cover layer would result in the cover layer flaking off locally. Corrosion would then take place uncontrollably, and there would be the risk in particular that corrosion might be induced in the areas of the implant subject to especially great mechanical stress.